1. Field of the Invention
The present invention relates generally to aqueous dispersions and dispersion compounds that optionally include a filler. More specifically, the present invention relates to dispersions that are useful in the shoe industry.
2. Background Art
Many shoes incorporate stiffeners, such as toe boxes (a.k.a. toe cap, toe puff) and counters to retain the shape of the shoe throughout the active life of the shoe. A counter is a piece of heavy leather or other stiffening material inserted between the outside and the lining of the upper at the back part of the shoe. The purpose of the counter is to strengthen the back part of the shoe and to prevent it from sagging and losing its shape. A toe box is a stiffener used to maintain the shape of a shoe toe, preserve toe room within the shoe, and give reasonable protection to the wearer's foot.
Processes used for the manufacture of base sheets for stiffener production include filmic processes, impregnation processes, and sintering. The preferred process to employ often depends upon the desired stiffness of the finished product. For example, there are three types of toe boxes: rigid, flexible, and soft. Rigid toes include safety shoes. Flexible toes may be flexed by thumb pressure, yet have semi-rigid walls. Soft toes include those which are intended to merely preserve the toe form and give maximum comfort to the wearer. A filmic process may be preferred for a rigid safety shoe, as materials prepared by a filmic process are often more resilient compared to impregnated materials.
In a filmic process, a thermoplastic sheet can be extruded and laminated to a low weight textile (typically 20 g/m2), followed by hot melt coating of a polyurethane or polyamide adhesive. The function of the fabric is to provide a compatible layer between the extruded sheet and the adhesive layer. For example, in U.S. Pat. No. 4,814,037, a shoe stiffener is formed from multilayer coextruded plastics.
In a sintering process, a thermoplastic powder, such as a polycaprolactone or a PVC-polycaprolactone (e.g. TONE™, a linear polycaprolactone polyester available from The Dow Chemical Company) is applied onto a textile and sintered in an oven. For example, German Patent No. 2,621,195, discloses stiffening materials which are produced by melting powdered mixtures of plastics and fillers onto textile fabrics. Adhesives are then used to bind the stiffener to other parts of a shoe. In U.S. Pat. No. 4,717496, the polyester used in the stiffening composition has both binding and adhesive properties.
In an impregnation process, a textile is impregnated with an emulsion or with a polymer solution. Impregnation systems can be water based or solvent based, and include SBR, natural and polychloroprene latexes, and styrenic polymers. The compatibility of the stiffener, fabric, and any adhesives used during shoe construction can affect the resiliency (lasting), stiffness, and shape retention of the stiffener and the shoe. For example, in GB 935,001, a heat activable shoe stiffener blank formed as a fibrous base is loaded or impregnated with PS, PVC, PVA, or an SB copolymer resin that stiffens after exposure to a solvent or to a temperature of 100° C., yet remains softenable such that the material can be reshaped as needed. U.S. Pat. No. 4,507,357, further adds to the stiffening composition a plasticizer and an aminoplast. In both of these patents, low temperature processability was desired so as to protect other shoe parts from damage during manufacture. U.S. Pat. No. 6,391,380 discloses a stiffener composition which includes a latex forming resin and a powder adhesive polyester resin. To avoid softening during storage or exposure to moderate temperatures, U.S. Pat. No. 3,961,124 employs a stiffening composition having a crosslinkable resin and a peroxide initiator, thermosetting the stiffener during shoe manufacture.
Combinations of the above processes can also be used. For example, In U.S. Pat. No. 3,708,332, fabrics were impregnated with mixtures containing polystyrene and styrene-butadiene copolymers and coated with polycaprolactone to form shoe stiffener sheets.
Other patents related to stiffening compositions and shoe stiffeners include U.S. Pat. Nos. 3,284,872, 3,778,251, and 3,961,124; British Patents GB 2,102,851, 2,091,768, 2,024,279, 1,065,418, 1,062,498, and 1,036,790, and WO2003/000810, among others.
Impregnated systems incorporating SBR, SB, polychloroprene, or a blend of polychloroprene and natural latex are common, as noted above. Each of these systems has drawbacks such as poor resiliency, elasticity, cost, curing requirements, and allergic properties, as well as potentially poor compatibility with other components in the shoe structure. In view of these drawbacks, there exists a need for suitable replacements for or alternatives to conventional latex stiffening compositions.